1. Field of Invention
The invention relates to kickback voltage reduction circuits and more particularly to a drive circuit incorporating one or two comparators adapted to sense kickback voltage generated in an inductive load and conduct two field-effect transistors connected to ground in a very short period of time so as to quickly reduce the kickback voltage.
2. Description of Related Art
Many inductive load drive systems for disk drive, fan motor drive or the like employ a drive circuit for reducing kickback voltage generated therein. A kickback voltage is generated when a current flowing through an inductive load is suddenly changed. The kickback voltage can apply to the drive system (i.e., circuit) to undesirably increase power consumption.
FIG. 1 shows a conventional drive circuit 5 for reducing kickback voltage generated in an inductive load. The circuit 5 comprises an H-shaped transistor assembly 1 including transistors (e.g., field-effect transistors (FETs)) 111, 112, 121, 122 and diodes 131, 132, 133, 134 in which the transistor 111 and the diode 131 are controlled by a logic control 211, the transistor 112 and the diode 132 are controlled by a logic control 212, the transistor 121 and the diode 133 are controlled by a logic control 221, and the transistor 122 and the diode 134 are controlled by a logic control 222 respectively. The circuit 5 operates as follows:
In FIG. 2, initially current I flows from the transistor 121 to the transistor 112 via an output OUT1, an inductive load (i.e., direct current motor) 3, and an output OUT2 when the transistor 121 is conducted by the logic control 221, the transistor 112 is conducted by the logic control 212, the transistor 122 is cut off by the logic control 222, and the transistor 111 is cut off by the logic control 211 respectively.
In FIG. 3, just before phase change recirculation current I flows from the transistor 111 to the transistor 112 via the output OUT1, the load 3, and the output OUT2 when the transistor 121 is cut off by the logic control 221, the transistor 112 is conducted by the logic control 212, the transistor 122 is cut off by the logic control 222, and the transistor 111 is conducted by the logic control 211 respectively.
In FIG. 4, recirculation current I flows from the transistor 111 to the transistor 122 via the output OUT1, the load 3, and the output OUT2 when phase changes when the transistor 121 is cut off by the logic control 221, the transistor 112 is cut off by the logic control 212, the transistor 122 is cut off by the logic control 222, and the transistor 111 is conducted by the logic control 211 respectively.
There is no additional component for directing current flowing from internal power source PVCC to ground GND other than a Zener diode 43 and a capacitor 42. In detail, current I will only flow from the capacitor 42 to ground if no Zener diode is provided. It is typical for the capacitor 42 having a small capacity. Thus, it is impossible for the capacitor 42 to store all electric charge discharged by the load 3. This in turn may direct the electric charge not stored by the capacitor 42 to both PVCC and OUT2. As a result, voltage of each of PVCC and OUT1 increases abruptly. This is called kickback voltage. Also, voltage difference between two terminals of the capacitor 42 increases continuously as voltage of each of PVCC and OUT1 continues to increase. Hence, the capacitor 42 can store more electric charge. Eventually, the capacitor 42 stores all electric charge discharged by the load 3. However, the kickback voltage may exceed the maximum voltage allowed by other components of the circuit 5, resulting in a damage to these components.
Voltage of OUT1 or OUT2 will increase as kickback voltage is generated. Eventually, the diodes 133, 134 are conducted to maintain the current flow from OUT2 to OUT1 via the load 3. Additionally, a Schottky Barrier Diode (SBD) 41 is provided to interconnect power supply VCC and PVCC. The provision of SBD can prevent reverse current from damaging VCC.
The graph of FIG. 11 corresponds to the addition of the SBD 41. In detail, the upper graph represents the curve of VCC. The intermediate graph represents the curve of PVCC when kickback voltage is generated. The lower graph represents the curve of OUT1 or OUT2 when the SBD 41 enables to clamp down the voltage of OUT1 or OUT2 in response to kickback voltage. As a result, the voltage level of OUT1 or OUT2 decreases greatly. This has the benefit of preventing VCC from being adversely affected by kickback voltage.
In addition, a Zener diode 43 is added to interconnect PVCC and ground GND. The increasing kickback voltage will increase voltage of OUT1 or OUT 2 and also make the Zener diode 43 to break down reversely. As a result, electric charge not stored by the capacitor 42 will be directed to GND via the Zener diode 43 and voltage of OUT1 or OUT2 will be clamped to a predetermined level.
The graph of FIG. 12 corresponds to the addition of the Zener diode 42. In detail, the upper graph represents the curve of VCC. The intermediate graph represents the curve of PVCC when kickback voltage is generated. The lower graph represents the curve of OUT1 or OUT2 when the Zener diode 42 enables to clamp down the voltage of OUT1 or OUT2 in response to kickback voltage. As a result, the voltage level of kickback voltage decreases greatly. This has the benefit of protecting other components of the circuit 5.
However, it is not always possible of integrating a Zener diode in a chip because many semiconductor manufacturing processes do not support such technology. For allowing large current to pass a Zener diode, the Zener diode is required to occupy a large area of a chip. This can result in an increase in the chip manufacturing cost. Moreover, a power supply VCC is available to have a wide range of voltage from about 2V to about 200V. Typically, two rules should be followed when selecting a power supply VCC as detailed below.
First rule: Reverse-breakdown voltage of a Zener diode should be higher than the maximum voltage of power supply VCC. Second rule: Reverse-breakdown voltage of a Zener diode should be lower than the maximum voltage allowed by other components of a circuit.
The first rule aims at preventing a power supply VCC from being reverse-breakdown. Otherwise, a quiescent current of a circuit may be adversely affected. The second rule aims at protecting other components in the circuit when kickback voltage is generated.
The conventional drive circuit 5 for reducing kickback voltage functions based on different voltage levels of power supply VCC and operating voltages of other components in the circuit. Also, the addition of a Zener diode has the drawbacks of greatly increasing the chip manufacturing cost.
There have been numerous suggestions in prior patents for kickback voltage reduction circuit. For example, U.S. Pat. No. 5,896,117 discloses such a circuit. Thus, continuing improvements in the exploitation of inductive kickback voltage reduction circuit are constantly being sought.